1. Field of the Invention
The present invention relates to a dielectric duplexer using a transverse magnetic (hereinafter referred to as “TM”) multiplex mode dielectric resonator and to a communication apparatus comprising the dielectric duplexer.
2. Description of the Related Art
In typical dielectric duplexers comprising a plurality of two or more types of TM mode resonators having different degrees of multiplexing is constructed such that the filter on the transmission side has certain passing characteristics and the filter on the receiving side has certain passing characteristics different from the filter on the transmission side. In such dielectric duplexers, a combination of a plurality of TM mode resonators form the filter on the transmission side and a combination of a plurality of TM mode resonators form the filter on a receiving side.
However, in dielectric duplexers comprising a plurality of such conventional TM mode resonators, there are problems to be solved which are described below.
In general, in dielectric duplexers comprising TM mode resonators, in order to decrease the external size of a duplexer, a dielectric duplexer is formed using a triple mode resonator.
However, in a communication apparatus having incorporated therein a dielectric duplexer comprising TM mode resonators, high power characteristics are often required on the transmission side of the dielectric duplexer. Depending on the input power thereof, if a resonator having a high degree of multiplexing is used, the current density is increased, and characteristics are deteriorated due to generated heat.
On the other hand, the quality factor Q of the triple mode resonator deteriorates by approximately 20% to 30% compared to a double mode resonator. For this reason, when a low insertion loss is required, loss is increased when the degree of multiplexing of the resonator which forms a filter is high.
In order to solve these problems, the degree of multiplexing of the resonator which forms a dielectric duplexer must be decreased. With this, however, the number of resonators is typically increased, thereby increasing the size of the duplexer and the cost.
In order to obtain each of the above-described required characteristics, a method of decreasing the degree of multiplexing of only one of the filters on the transmission side and on the receiving side may be considered. However, since the external dimensions differ between a double mode resonator and a triple mode resonator, the sharing parts between the transmission side and the receiving side is difficult, and the cost is increased. For example, in order to form a triple mode resonator using TM110x+y, TM110x−y, and TM111 modes, and a double mode resonator using TM110x and TM110y modes (or a double mode resonator using TM110x+y and TM110x−y modes) from the same material such that they operate at the same frequency band, the external dimensions of each become different. Specifically, for example, in the 1.8-GHz band, when a material having a specific inductive capacity εr of 24 is used to form the respective resonators, the triple mode resonator is formed in a square of approximately 25 mm, and the double mode resonator is formed in a square of approximately 35 mm.
Also, where resonators having different degrees of multiplexing are mixed inside a filter on the transmission or the receiving side (for example, to form a filter of seven stages, i.e., two double mode resonators and a triple mode resonator), the external dimensions of the respective parts are not uniform, it is difficult to use parts in common between the transmission side and the receiving side, and the cost is increased. Further, since the external dimensions of the parts differ, an unnecessary space is created, and thus the space within the communication apparatus cannot be fully utilized.
Furthermore, when a combination of TM mode resonators in which the characteristics of a filter on the transmission side and a filter on the receiving side is used with the same specification, the outside shape of the duplexer becomes uniform. However, one of filters on the transmission side and one of the filters on the receiving side is sometimes formed as a filter with more stages than are necessary in terms of required characteristics. As a result, the filter has excessive attenuation characteristics, and becomes inferior to an ideal design in terms of insertion loss. Therefore, it is not possible to simultaneously accomplish a reduced size and a lower loss.